48f> Colours of Stars. 



Eamsden construction, a slip of glass micrometrically divided, 

 project its image, together with that of the sun, as a scale upon 

 the screen. Ihe latter gives the following dimensions, which 

 may be useful as a guide : — Telescope 8| inches aperture, iu-u 

 darkened room; power 80; cardboard screen on easel, 4 feet 

 2 inches from eye-piece; glass micrometer in focus divided to 

 200ths of an inch, each division giving about \ inch on screen, 

 where a corresponding scale is drawn with ink, every 16th of 

 an inch representing about 4". With other powers, other dis- 

 tances would be required for the screen. With a good tele- 

 scope, magnifying may, of course, be pushed much further; 

 but beyond 80, or at the most 90, the field would probably fail to 

 admit the whole disc of the sun. Captain Noble states that he 

 obtains extremely beautiful views of the solar phenomena by 

 fitting on to the eye-piece the small end of a cardboard cone, 

 1 foot long, and 6 inches across the larger end, which is filled 

 by a disc of plaster of Paris, carefully smoothed while wet on a 

 sheet of plate-glass ; on this the image is projected, the in- 

 terior of the cone being blackened, and an opening cut in its 

 side to view the face of the plaster screen. 



The observer by direct vision will not be surprised if he 

 should find a different focus required for spots in the centre 

 and those near the limb. This is a remark of long standing, 

 but the direction of change has not been always accordantly 

 given. Harding (Schroter's assistant) thought the focus 

 shorter for the marginal than the central spots; Gruithuisen, 

 Dawes, and Hind the reverse. Gruithuisen ingeniously ascribed 

 this peculiarity to a negative refraction in the solar atmos- 

 phere ; * Dawes, more soundly, to the effect upon the eye of 

 the different intensity of light in the two regions, which is very 

 considerable ; and this view is confirmed by similar obser- 

 vations . that ho has made upon other objects, such as the 

 brighter and darker portions of the moon, or the planet 

 Saturn. 



COLOURS OP STARS. 



The diversity of colour among the stars is a fact which is 

 apparent upon even a very cursory survey of the heavens. 

 To .some eyes it is probably much more evident than to 

 others; fche strange phenomenon of " colour blindness," — in 

 other words, a defective or incorrect appreciation of difference 

 of hue — being, it is said, more common than is usually sup- 

 posed. Still, the generality of spectators would at any rate be 

 struck with the moro extreme cases; for instance, a compa- 



* By a curious coincidence, Sir J. HerBchel and Mr. Hunt, in the early days 

 "I photography, were led to conclude that a c-Iubs of rays having peculiar negative 

 actinic properties issue from the edges of the sun. 



